The well-known fact that cobalt(II) halide complexes in various media undergo changes in coordination, absorbance, and color with increasing temperature was combined with the high latent heat of fusion of new materials suitable for simultaneous control of temperature, light intensity, and heat storage. Hence, the increase in absorbance with temperature acts as an autoregulated shading protection from overheating. Several binary mixtures of calcium nitrate tetrahydrate and ammonium nitrate, xCa(NO(3))(2)center dot 4H(2)O + (1 - x)NH(4)NO(3), with low melting points were studied as suitable solvents for the formation of cobalt(II) chloride complexes. The temperature dependence of the densities of these melts has been measured. Absorption spectra in the visible spectral range [(400 to 800) nm] have also been investigated. The overall stability constants of [Co(NO(3))(4)](2-), [Co(NO(3))(2)Cl(2)](2-), and [CoCl(4)](2-) complexes at different temperatures were calculated using a nonlinear regression program. On the basis of these stability constants, the thermodynamic data (i.e., enthalpy, entropy, and free-energy changes) were calculated for the corresponding complexation reactions. Also, the dependence of the enthalpy and entropy changes on the solvent composition has been analyzed.